The present invention relates generally to implantable expandable medical devices and more particularly to implantable endoluminal stents, covered-stents, stent-grafts and grafts employed to restore and maintain patency of anatomical passageways within a mammalian body. The dramatic success enjoyed by a variety of endoluminal implantable medical devices has largely been the result of their ability to be delivered utilizing minimally invasive techniques that significantly reduce the trauma to the patient. While there are many types of endoluminally-delivered implantable medical devices, the present invention relates specifically to generally tubular devices that are expandable from a first smaller diameter suitable for minimally invasive delivery to a second enlarge diameter suitable for restoring and maintaining patency of the anatomical passageway.
The most prevalent type of implantable expandable endoluminal device is the stent. Stents are typically used to treat occlusive and anuersymal disease or trauma and are, typically, generally tubular structural scaffolds typically consisting of latticed arrays of circumferential members and longitudinal members. The circumferential members typically permit the stent to radially expand from the first to the second diameter while the longitudinal members provide column strength and longitudinal flexibility. A variant of the stent, termed in the art either a stent-graft or covered stent, consists of a stent or other structural scaffold covered with a graft. Stent-grafts are devices typically employed for exclusionary purposes for purposes of creating a conduit, such as in excluding an abdominal aortic aneurysm, whereas covered stents are devices typically employed in treating occlusive conditions, such as coronary artery disease to restore patency to the coronary artery. Conventional stent-grafts and covered stents employ polymeric covers, such as polyester or expanded polytetrafluoroethylene that are either affixed to the stent by barbs or sutures or are retained on the stent by adhesion either to the stent or to an opposing graft surface.
Recently, radially expandable metallic grafts have been disclosed in co-pending, commonly assigned U.S. patent application Ser. Nos. 10/135,316 and 10/135,626, both filed Apr. 29, 2002, both of which are hereby expressly incorporated by reference, disclose a nitinol thin film graft having a pattern of microperforations that permit radial enlargement of the graft by geometric deformation of the microperforations.
Cardiovascular devices, in particular, should maintain vascular patency and prevent re-occlusion of the vascular system. There are two main types of implantable cardiovascular devices that are designed to restore and maintain vascular patency. The first type is a balloon expandable device that requires application of an externally applied force in order to radially distend the device. The second type is a self-expanding device that radially distends based upon inherent mechanical or material properties of the device, requires removal of a constraining force, and does not require application of an external force for radial distension. There are two main sub-types of self-expanding devices. A first sub-type consists of devices fabricated from traditional elasto-plastic materials, such as 316L stainless steel, that elastically recover from a constraining force applied to maintain the device in a smaller delivery diameter. The normalized strains typically associated with such traditional elasto-plastic materials is typically less than 1%. The second sub-type consists of devices fabricated from shape memory or superelastic materials that recover their shape under defined thermo-mechanical conditions. The recoverable strains associated with these materials can significantly excel 1%.
It is ironic that a major problem with existing self-expanding stents and their designs lies in their inherent spring properties. Current self-expanding stents are de facto elastically deformable along their longitudinal axis. In this manner, the device is capable of longitudinal bending and traversing the tortuous endoluminal pathways required to place the device at its intended in vivo site. Yet the very elastic nature of these devices imposes an inherent spring bias to the device which favors a zero-strain state in the normal or linear axial configuration. Thus, when longitudinally flexed, conventional self-expanding stents exhibit a positive strain and seek to return to the unbent or zero-strain normal conformation. When such a device is implanted into a non-linear vessel, in order to conform to the geometry of the vessel, the implanted device is in a strained longitudinal conformation that exerts continual, unevenly distributed stress against the vascular walls. This continual stress exerted on the vascular wall may ultimately lead to vascular injury and threaten clinical outcome.
Conventional stent designs have sought to control the inherent longitudinal spring bias by altering the geometric pattern of the structural components of the stent. Heretofore, however, there has been little focus in the art on altering the mechanical properties of selected regions of the material used to fabricate the implantable device. By selectively altering the mechanical properties of the device material, the device can have regions intended for plastic or pseudoplastic deformation and regions intended for elastic or pseudoelastic deformation. Thus, in accordance with the present invention there is provided within a single self-expanding device regions that are differentiated by their mechanical properties according to their intended functionality.
The term “elastic deformation,” as used herein, is defined as a deformation of a traditional metal material caused by an applied load that is completely recoverable upon removal of the applied load. The elastic limit of a traditional metal is typically less than 1% strain.
The term “plastic deformation,” as used herein, is defined as deformation of a traditional metal material caused by an applied load that cannot be completely recovered upon removal of the load because bonds have been broken.
The term “elasto-plastic,” as used herein, is intended to mean materials that are capable of both elastic deformation and plastic deformation.
The term “pseudoelastic deformation,” as used herein, is defined as a deformation caused by an applied load that is completely recoverable upon removal of the load and the limit of which is characterized by being significantly larger than the elastic limit of a traditional metal (8% strain in the case of nitinol). This phenomenon is caused by a load or stress induced solid-state phase change that is reversible upon removal of the load.
The term “pseudoplastic deformation,” as used herein, is defined as a deformation caused by an applied load that requires some other action besides load removal, such as the application of heat, for complete recovery of the deformation. In pseudoplastic deformations, bonds have not been broken but, instead, have been reoriented or detwinned in the case of martensitic nitinol.
As used herein, the term “pseudometal” and “pseudometallic material” is defined as a biocompatible material which exhibits biological response and material characteristics substantially the same as biocompatible metals. Examples of pseudometallic materials include, for example, composite materials, ceramics, quartz, and borosilicate. Composite materials are composed of a matrix material reinforced with any of a variety of fibers made from ceramics, metals, or polymers. The reinforcing fibers are the primary load carriers of the material, with the matrix component transferring the load from fiber to fiber. Reinforcement of the matrix material may be achieved in a variety of ways. Fibers may be either continuous or discontinuous. Reinforcement may also be in the form of particles. Examples of composite materials include those made of carbon fibers, boron fibers, boron carbide fibers, carbon and graphite fibers, silicon carbide fibers, steel fibers, tungsten fibers, graphite/copper fibers, titanium and silicon carbide/titanium fibers.
A stress-strain curve for austenitic nitinol in which a sample is taken all the way to failure at a temperature above Af (finish of Austenitic transformation) can be separated into the following regions: elastic deformation of austenite, pseudoelastic deformation of austenite to stress induced martensite, elastic deformation of the stress induced martensite, plastic deformation of the stress induced martensite and fracture. Removal of the load at any point before the onset of plastic deformation of the stress induced martensite will result in complete recovery of the deformation.
Nitinol is in the thermally-induced martensite state if the material is brought to a temperature below Mf (finish of martensitic transformation) and subsequently kept below As (onset of austenitic transformation). If the material is sufficient deformed (greater than 0.5% strain) while in its thermally-induced martensitic state and subsequently constrained at temperatures above As it is still considered to be in its thermally-induced martensite state and not in a stress-induced martensite state. A stress-strain curve for martensitic nitinol in which a sample is taken all the way to failure at a temperature below As can be separated into the following regions: elastic deformation of thermally induced martensite, pseudoplastic deformation of thermally induced martensite via detwinning, elastic deformation of the detwinned thermally induced martensite, plastic deformation of the detwinned thermally induced martensite and fracture. Removal of the load at any point before the onset of plastic deformation of the detwinned thermally induced martensite will result in complete recovery of the deformation when heated above Af.
A standard procedure employed in the art to control the temperature-dependent mechanical properties of nickel-titanium alloys is to employ precipitation heat treatment. Heat treatment of a formed nickel-titanium device by subjecting the device to temperatures between 300-500° C. forces nickel-rich precipitates out of solution, i.e., out of the grains and into the intergranular boundaries. The result is a net increase in the titanium content of the grains and a concomitant increase in the transition temperature of the device. The use of precipitation heat treatment to selectively alter the mechanical properties of regions of a device based upon intended functionality has not, heretofore, been employed. Additionally, the use of vacuum deposition to control local bulk chemical content in precursor blanks and formed devices based upon intended functionality of the device has, heretofore, been unknown in the art of medical device fabrication.